Electrochromic devices and methods

ABSTRACT

Electrochromic devices are disclosed which may be used for large surface area applications. The devices utilize optical tuning to minimize optical interference between layers of the structure and to maximize uniform optical transparency. Optical tuning also enables transparent conductive oxide layers to be replaced by thin conductive metal layers, thereby reducing the overall thickness of these devices and facilitating the manufacturing process.

This is a continuation of application Ser. No. 07/996,676 filed Dec. 24,1992, U.S. Pat. No. 5,485,303 which is a continuation-in-part of Ser.No. 07/754,650 filed on Sep. 4, 1991, now U.S. Pat. No. 5,321,544.

FIELD OF THE INVENTION

The present invention relates to electrochromic devices through whichenergy, including light, can be transmitted under controlled conditions.More particularly, the present invention is directed to improvedelectrochromic devices which are usable over large surfaces, as well asmethods for manufacturing these devices.

BACKGROUND OF THE INVENTION

Certain materials, referred to as electrochromic materials, are known tochange their optical properties in response to the application of anelectric current or an electric potential. This property has been takenadvantage of to produce electrochromic devices which can be controlledto transmit optical energy selectively. Such electrochromic devicestypically have a structure consisting of sequential layers including alayer of an electrically conducting material, an electrode formed from alayer of an electrochromic material, an ion conductive layer, acounterelectrode layer, and another electrically conductive layer. In afirst condition of the electrochromic device, each of the aforementionedlayers is optically transparent such that a majority of the opticalenergy incident on the device will be transmitted therethrough. Upon theapplication of an electric potential across these layers, however, theoptical properties of the electrochromic material will change such thatthe electrochromic layer will become less transparent, therebypreventing the transmission of much of the optical energy.

One of the most significant potential uses of these electrochromicdevices is to control the transmission of optical energy throughwindows, and particularly the large windows of office buildings andother such structures. By selectively controlling the transmission ofoptical energy through these windows, tremendous cost savings in termsof heating and cooling these buildings can be realized. To date,however, efforts to capitalize on this potential benefit have largelyfailed. One reason for this failure has been the inability of theindustry to produce economically an electrochromic device that can beused effectively over large surfaces. Those devices which have beenavailable heretofore have exhibited an undesirable mosaic of differentcolors or an iridescent affect when used over large surface areas. Sincethe color of these devices is directly related to the thicknesses of thevarious layers, variations in layer thickness result in color changesfrom region to region, thereby producing this iridescent affect. Theextremely tight tolerances which are required to overcome thisdifficulty make the use of these devices for large surface areaapplications largely uneconomical.

A further obstacle to the widespread commercialization of large surfacearea electrochromic devices relates to the understanding in this fieldthat optically transparent conductive materials must be used to apply anelectric potential across these devices so that the optical transparencyof these devices will not be compromised. Preferably, the conductivelayers formed from these materials have a low sheet resistance in orderto effect a sufficiently rapid and uniform change in optical propertiesthroughout the device. These layers are typically formed from costlytransparent conductive metal oxides. The relatively thick layers ofthese oxides needed to get an acceptably low sheet resistance for largesurface area devices tends to decrease their overall transparency.Moreover, in view of the high cost of these materials, the relativelylarge quantities required makes the use of these materials uneconomical.

There therefore exists a need for improved electrochromic devices whichwill provide an acceptable appearance when used over large surfaceareas. There exists a further need for such electrochromic devices whichcan be produced efficiently and economically, and which therefore willbe available for widespread commercial use.

SUMMARY OF THE INVENTION

The present invention addresses these needs.

One aspect of the present invention provides an electrochromic deviceconsisting of an electrochromic structure including an electrode formedfrom an electrochromic material, a counterelectrode and transportingmeans for transporting ions between the electrode and thecounterelectrode. Conductive means including at least two electricallyconductive layers sandwich the electrochromic structure so that anelectric potential can be applied to the structure. The electrochromicdevice further includes enhancing means for enhancing the transmissionof radiation through at least one of the electrically conductive layers.Preferably, the electrode, the counterelectrode and the ion conductivelayers are selected to all have about the same index of refraction.

In preferred embodiments, the enhancing means comprises at least onelayer of an optically transparent material in surface contact with theat least one electrically conductive layer. Preferably, the opticallytransparent material is a transparent oxide, a transparent nitride, or acombination thereof. In some highly preferred embodiments, the opticallytransparent material comprises a mixture of silicon oxide and tin oxide.

The transporting means desirably includes at least one layer formed froman ion conducting material sandwiched between the electrode and thecounterelectrode. In these embodiments, the electrochromic material ispreferably formed from tungsten oxide, niobium oxide, titanium oxide,molybdenum oxide, nickel oxide, iridium oxide, or mixtures thereof. Thecounterelectrode is preferably formed from vanadium oxide, niobiumoxide, indium oxide, nickel oxide, cobalt oxide, molybdenum oxide ormixtures thereof. The ion conducting material may be a lithium ionconducting material such as lithium silicate, lithium borosilicate,lithium aluminum silicate, lithium niobate, lithium nitride, or lithiumaluminum fluoride. Alternatively, the ion conducting material may be ahydrogen ion conducting material such as silicon dioxide or tantalumpentoxide. Preferably, the electrode, the counterelectrode and the ionconducting layer are selected to all have about the same index ofrefraction. This may be accomplished by adjusting the index ofrefraction of these materials by additions of oxides having differentindices of refraction.

In one embodiment in accordance with this aspect of the invention, theelectrochromic device further includes a substrate in surface contactwith the at least one layer of an optically transparent material, thesubstrate having a first index of refraction, the at least oneelectrically conductive layer having a second index of refraction, andthe at least one layer of an optically transparent material having athird index of refraction between the first and second indices ofrefraction. In preferred electrochromic devices in accordance with thisembodiment, the index of refraction of the at least one layer of anoptically transparent material is about equal to the square root of themathematical product of the first and second indices of refraction.Also, the at least one layer of an optically transparent materialpreferably has a thickness which is inversely proportional to the indexof refraction of the at least one layer of an optically transparentmaterial, and more preferably between about 60 nm and about 90 nm.

In another embodiment, the enhancing means may consist of a series ofoptically transparent layers beginning with a first opticallytransparent layer in surface contact with the at least one electricallyconductive layer and ending with an nth optically transparent layer.This embodiment of the device may further consist of a substrate insurface contact with the nth optically transparent layer, the at leastone electrically conductive layer having a first index of refraction andthe substrate having a second index of refraction less than the firstindex of refraction. In this embodiment, the index of refraction of thefirst optically transparent layer is less than the first index ofrefraction, the index of refraction of the nth optically transparentlayer is greater than the second index of refraction, and the index ofrefraction of each one of the series of optically transparent layersdecreases monotonically from the index of refraction of the firsttransparent layer to the index of refraction of the nth transparentlayer.

In still another embodiment, the enhancing means may consist of twooptically transparent layers, one of the optically transparent layersbeing in surface contact with the at least one electrically conductivelayer and another of the optically transparent layers being in surfacecontact with the one optically transparent layer. In this embodiment,the at least one electrically conductive layer has a first index ofrefraction, the one of the optically transparent layers has an index ofrefraction which is less than the first index of refraction, and theanother of the optically transparent layers has an index of refractionwhich is greater than the index of refraction of the one of theoptically transparent layers. In preferred embodiments of this device,the one of the optically transparent layers has an index of refractionbetween about 1.4 and about 1.7, and the other of the opticallytransparent layers has an index of refraction about equal to or greaterthan the first index of refraction. Further, the two opticallytransparent layers of preferred devices in accordance with thisembodiment have a combined thickness of between about 30 nm and about 70nm.

In yet another embodiment, at least one of the electrically conductivelayers includes a layer of a first electrically conductive metal.Preferably, this first electrically conductive metal layer has athickness of between about 5 nm and about 15 nm, and more preferablybetween about 7 nm and about 12 nm. Optionally, this embodiment mayfurther include an intermediate layer disposed between theelectrochromic structure and the first electrically conductive metallayer. This intermediate layer may be formed from a second electricallyconductive metal, and preferably a metal which is more stable than thefirst electrically conductive metal, or from an electrically conductiveoxide.

Preferably, the enhancing means in this last embodiment includes atleast one layer of an optically transparent material in surface contactwith the first electrically conductive metal layer. Still morepreferably, the optically transparent material comprises an electricallyconductive oxide. Preferred optically transparent materials have anindex of refraction greater than about 1.9, and desirably between about1.9 and about 2.8, and a thickness of between about 10 nm and about 50nm, and more preferably between about 23 nm and about 45 nm.

In still another embodiment of the present invention, at least one ofthe electrically conductive layers includes a layer of an electricallyconductive oxide, and preferably an oxide having about the same index ofrefraction as the electrode, the counterelectrode and the ion conductinglayer. Electrochromic devices in accordance with this embodiment mayfurther include a layer of an electrically conductive metal in surfacecontact with the electrically conductive oxide layer. In preferredembodiments, the electrically conductive oxide layer is disposed betweenthe electrochromic structure and the conductive metal layer. Desirably,the enhancing means in these embodiments includes at least one layer ofan optically transparent material having an index of refraction greaterthan about 1.9 in surface contact with the conductive metal layer.Preferably, these optically transparent layers have a thickness ofbetween about 10 nm and about 50 nm.

A further aspect of the present invention provides an electrochromiccombination consisting of a transparent substrate and an electrochromicdevice arranged on the transparent substrate. The electrochromic devicemay generally consist of an electrochromic structure including anelectrode formed from an electrochromic material, a counterelectrode andtransporting means for transporting ions between the electrode and thecounterelectrode. The electrochromic device may further consist ofconductive means including at least two electrically conductive layerssandwiching the electrochromic structure for applying an electricpotential to the electrochromic structure, and enhancing means forenhancing the transmission of radiation through at least one of theelectrically conductive layers.

Preferred embodiments in accordance with this aspect of the presentinvention may further include a transparent superstrate, wherein theelectrochromic device is sandwiched between the transparent substrateand the transparent superstrate. A layer of an adhesive material may beused to bind the transparent superstrate to the electrochromic device.Such adhesive layer preferably has an index of refraction between about1.4 and about 1.8, which index of refraction is preferably substantiallyequal to the index of refraction of the superstrate. Optionally, the atleast one electrically conductive layer may include a layer of anelectrically conductive oxide, and the adhesive layer may be in surfacecontact with the electrically conductive oxide layer. In that event, theadhesive layer preferably has an index of refraction between the indexof refraction of the electrically conductive oxide layer and the indexof refraction of the superstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentinvention and the various advantages thereof can be realized byreference to the following detailed description, in which reference ismade to the accompanying drawings in which:

FIG. 1 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with the prior art;

FIG. 2 is a graphical representation showing the optical transmission ofthe electrochromic device of FIG. 1 in the bleached state;

FIG. 3 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with one embodiment of the present invention;

FIG. 4 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with another embodiment of the present invention;

FIG. 5 is a graphical representation showing the optical transmission ofthe electrochromic device of FIG. 4 in the bleached state;

FIG. 6 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with a further embodiment of the present invention;

FIG. 7 is a graphical representation showing the optical transmission ofthe electrochromic device of FIG. 6 in the bleached state;

FIG. 8 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with still another embodiment of the presentinvention;

FIG. 9 is a graphical representation showing the optical transmission ofthe electrochromic device of FIG. 8 in the bleached state;

FIG. 10 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with yet another embodiment of the presentinvention; and

FIG. 11 is a highly schematic cross-sectional view of an electrochromicdevice in accordance with a still further embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purpose of the present description, the electrochromic devicesof the present invention are discussed in connection with their use tocontrol the transmission of light through a window. It will beappreciated, however, that these electrochromic devices are useful in awide variety of applications, including display devices, variablereflectance mirrors, lenses and similar devices in which the ability toselectively control the transmission of optical energy through atransparent structure would be beneficial.

A window 10 incorporating an electrochromic device in accordance withthe prior art is shown schematically in cross-section in FIG. 1. Window10 consists of a series of sequential layers, including a transparentglass substrate 12, a transparent conductive oxide layer 20, anelectrochromic electrode layer 30, an ion conducting layer 40, acounterelectrode layer 50, another transparent conductive oxide layer22, and a transparent glass superstrate 14. A low voltage battery 2 andswitch 4 are connected to the layered structure by means of conductivewires 6 and 8. In order to alter the optical properties of window 10,switch 4 is closed whereupon battery 2 will cause an electric potentialto be created across the layered structure. The polarity of the batterywill govern the nature of the electric potential created and thus thedirection of ion and electron flow. In the embodiment shown in FIG. 1,the electric potential created as switch 4 is closed will cause ions toflow from the counterelectrode layer 50 through the ion conducting layer40 to the electrochromic electrode layer 30, thereby reducing theelectrochromic material to its so-called "colored" state. In this state,the transparency of window 10 is substantially reduced as a largeportion of the optical energy incident on window 10 is absorbed andreflected by the electrochromic electrode layer 30. Window 10 is said tohave a "memory" in that electrochromic layer 30 will remain in thiscolored state even when switch 4 is opened, provided layer 40 is alsoelectrically insulating. However, when the polarity of battery 2 isreversed and switch 4 is closed, the applied electric potential willcause ions to flow in a reverse direction from the electrochromicelectrode layer 30 through the ion conducting layer 40 to thecounterelectrode layer 50, thereby oxidizing the electrochromic materialto its so-called "bleached" state in which the transparency of window 10is at a maximum.

In fabricating window 10 described above, layers 20 and 22 may be formedfrom any transparent oxides which are highly electron conducting, suchas doped tin oxide, doped zinc oxide, tin-doped indium oxide and similarmaterials. The materials for forming layers 20 and 22 need not be thesame. Electrochromic electrode layer 30 is typically formed from amaterial whose optical properties can be reversibly altered as its stateof oxidation changes. The thickness of electrochromic electrode layer 30will normally be such that, in the colored state, an acceptablereduction in the transparency of the window is obtained. A widely usedmaterial in this regard is tungsten oxide (WO₃), although other suitablematerials may be used, such as molybdenum oxide, nickel oxide, iridiumoxide, niobium oxide, titanium oxide and mixtures of the foregoingoxides. The ion conducting layer 40 is used to transport ions into andout from the electrochromic layer 30, and must exhibit and maintain twoelectrically opposed properties. That is, ion conducting layer 40 mustreadily transmit ions upon the application of an electric potential, yetmust remain electrically insulating with respect to the transmission ofelectrons. In that regard, ion conducting layer 40 must have a thicknesssufficient to avoid the possibility of electron arcing or shortingbetween the electrochromic electrode layer 30 and the counterelectrodelayer 50. Suitable materials for forming layer 40 for the transmissionof lithium ions include, for example, lithium silicate, lithiumborosilicate, lithium aluminum silicate, lithium niobate, lithiumnitride and lithium aluminum fluoride; and suitable materials fortransmitting hydrogen ions include tantalum pentoxide and silicondioxide. Alternatively, ion conducting layer 40 may be formed from apolymer material.

The counterelectrode layer 50 of window 10 is typically formed from amaterial which is capable of storing ions and then releasing these ionsfor transmission to electrochromic layer 30 in response to anappropriate electric potential. The thickness of this layer ispreferably such that the counterelectrode is capable of transmitting alarge enough quantity of ions to the electrochromic electrode layer toeffect in that layer an acceptable change in color. Somecounterelectrode materials are themselves also electrochromic in thattheir optical properties also change as they give up or receive ions inresponse to the application of an electric potential. Theseelectrochromic counterelectrode materials may complement the affect thatan electric potential has on the optical properties of theelectrochromic electrode materials. That is, these counterelectrodematerials may become less transparent as they release ions to convertthe electrochromic materials to the colored state. Similarly, thecounterelectrode materials may become more transparent as they receiveions upon the conversion of the electrochromic material to the bleachedstate. Suitable materials for forming counterelectrode layer 50 includevanadium oxide, niobium oxide, indium oxide, nickel oxide, cobalt oxide,molybdenum oxide and mixtures of the foregoing oxides.

Each of the layers described above may be deposited by known techniques,provided that discrete and continuous individual layers are formed. Theparticular method of deposition for each layer depends upon severalparameters, including the material being deposited, the thickness of thelayer being deposited, the materials deposited in previous layers, etc.Deposition techniques including RF sputtering, chemical vapordeposition, plasma enhanced chemical vapor deposition, electron beamevaporation, sol-gel techniques and other known methods for depositingthin films are typically used.

During the fabrication process, at least one of the electrochromicelectrode layer 30 and the counterelectrode layer 50 may be insertedwith appropriate ions, such as lithium or hydrogen ions, unless theseions are already present in one of these layers in its deposited form.Ion insertion may be accomplished by treating layer 30 or layer 50 witha suitable reducing agent. For example, n-butyl lithium may be used forlithium insertion, or aqueous sulfuric acid may be used for hydrogeninsertion. Alternatively, ion insertion may be accomplished by a vacuumprocessing step, such as sputtering from a target serving as a source ofsuitable ions, such as a lithium target which decomposes to producelithium atoms in the vapor phase. Also, hydrogen insertion may beaccomplished by exposure to a hydrogen plasma. Ion insertion may also beaccomplished electrochemically by reduction in an appropriateion-containing electrolyte. A still further technique is to deposit alayer of the reduced material directly by vapor deposition in a reducingatmosphere which will react with the source or target material to formthe desired composition, or by using a source or target having thereduced composition. Still another technique for insertion uses avolatile precursor and ignites a low vapor pressure discharge todissociate the insertion ion from the precursor. For example, an organiclithium compound in the form of a gaseous precursor may be dissociatedso that lithium ions come into contact with the material into which theyare to be inserted.

FIG. 2 shows the transmission of optical energy as a function ofwavelength in the bleached state for a window 10 in which glasssubstrate 12 has an index of refraction of 1.5, transparent conductiveoxide layer 20 has an index of refraction of 2.1 and a thickness of 450nm, electrochromic electrode layer 30 has an index of refraction of 2.1and a thickness of 300 nm, ion conducting layer 40 has an index ofrefraction of 1.5 and a thickness of 200 nm, counterelectrode layer 50has an index of refraction of 2.0 and a thickness of 200 nm, transparentconductive oxide layer 22 has an index of refraction of 2.1 and athickness of 450 nm, and glass superstrate 14 has an index of refractionof 1.5. The bleached stated is shown because it is more sensitive to thedifficulties of achieving uniform optical transmission as a function ofwavelength. As can be clearly seen, there is very little uniformity tothe optical transmission through this layered structure, resulting in awindow having a colored mosaic or iridescent affect.

One aspect of the present invention overcomes the economic disadvantageand reduces the optical difficulties associated with the use oftransparent conductive oxides in large surface area opticallytransparent electrochromic devices. An embodiment of a window 100incorporating an electrochromic device in accordance with this aspect ofthe present invention is shown in schematic cross-section in FIG. 3.Window 100 is similar to window 10 described above with the positions ofthe electrochromic electrode layer and the counterelectrode layerreversed. Thus, window 100 includes transparent glass substrate 12,transparent conductive oxide layer 20, counterelectrode layer 50, ionconducting layer 40, and electrochromic electrode layer 30. These layersare formed from substantially the same materials as described above inconnection with window 10. Rather than a second transparent conductiveoxide layer 22, however, window 100 has a combination of layersincluding a layer 21 formed from an electrically conductive metal and alayer 60 formed from a transparent oxide which serves as an opticaltuning layer. Optionally, window 100 may also include a barrier layer 80formed from an inert or stable electrically conductive metal, such asnickel, for preventing ions from migrating from electrochromic electrodelayer 30 to conductive metal layer 21, and a second layer 90 which maypromote adhesion of optical tuning layer 60 to conductive metal layer 21or may act as a barrier to prevent the dissolution of the conductivemetal in layer 21 into transparent oxide layer 60. The assembly iscompleted by a transparent glass superstrate 14.

In the above-described structure, layer 21 may be formed from silver oranother highly conductive metal, such as copper or aluminum. By using ahighly conductive metal, very thin layers having little opticalabsorption and low sheet resistance can be formed, thereby overcomingthe cost and manufacturing difficulties associated with the use ofrelatively thick layers of transparent conductive oxides. Preferably,layer 21 has a thickness of between about 5 nm and about 15 nm, and morepreferably between about 7 nm and about 12 nm. Even though very thinlayers of silver and other conductive metals are not highly opticallytransparent due to their high reflectivity, optical transmissiontherethrough may be improved by the presence of an appropriatelyselected optical tuning layer 60. Optical tuning layers, such as layer60, which are used in combination with conductive metal layerspreferably have a thickness between about 10 nm and about 50 nm, andmore preferably between about 23 nm and about 40 nm. Preferred materialsfor forming optical tuning layer 60 have an index of refraction which isgreater than about 1.9, and more preferably between about 1.9 and about2.8. When optical tuning layer 60 is formed with a thickness of about30-40 nm and with an index of refraction which is substantially the sameas or greater than that for counterelectrode 30 so that the conductivemetal layer 21 having a low index of refraction is sandwiched betweenlayers 60 and 30 having higher indices of refraction, the reflectedsignals at the various interfaces between layers 14, 60, 21, 30 and 40will be reduced, thereby lowering reflection and maximizingtransmission. As a result, window 100 will be largely opticallytransparent in the bleached state of the electrochromic electrode layer30.

Further improvements in the transmission of optical energy throughelectrochromic devices can be realized by using the principle of anoptical tuning layer, as described above, in combination withtransparent conductive oxide layers. A window 110 incorporating suchfeature is shown in schematic cross-section in FIG. 4. Window 110consists of a glass substrate 12, a transparent optical tuning layer 61,a transparent conductive oxide layer 20, an electrochromic electrodelayer 30, an ion conducting layer 41, a counterelectrode layer 50, atransparent conductive oxide layer 22, a transparent optical tuninglayer 62, a laminate layer 70, and a transparent glass superstrate 14.Each of layers 12, 20, 30, 50 and 14 are the same as described above inconnection with window 100, with the positions of the electrochromicelectrode layer 30 and the counterelectrode layer 50 reversed so thatthey are again in the orientation shown in FIG. 1. In this embodiment,conductive metal layer 21 and optical tuning layer 60 have been replacedwith a transparent conductive oxide layer 22 which may or may not beformed from the same transparent conductive oxide as layer 20, and anappropriate optical tuning layer 62. In addition, an optical tuninglayer 61 has been inserted to improve upon the optical transmissionthrough window 110, and more particularly, to improve upon theuniformity of the optical transmission as a function of wavelength.

The selection of appropriate optical tuning layers 61 and 62 is madewith reference to the equation

    index of refraction=n+ik

where n is the real component of the index of refraction, i is thesquare root of -1, and k is the imaginary component of the index ofrefraction related to absorption. In order to optimize the opticaltuning effect, layers 61 and 62 are selected so that they aretransparent to visible light (i.e., the imaginary component of the indexof refraction, k, is approximately equal to zero so there is littleabsorption), and the real component of the index of refraction of theselayers, n, is equal to the geometric mean of the indices of refractionof the layers on either side of these optical tuning layers. Thus,optical tuning layer 61 preferably is formed from a transparent materialhaving an index of refraction which is equal to the geometric mean ofthe index of refraction of glass substrate 12 and the index ofrefraction of transparent conductive oxide layer 20. Similarly, opticaltuning layer 62 preferably is formed from a transparent material havingan index of refraction which is equal to the geometric mean of theindices of refraction of conductive oxide layer 22 and laminate layer70. As used herein, the "geometric mean" refers to the square root ofthe mathematical product of the two indices of refraction.

Proper optical tuning is also a function of the thickness of opticaltuning layers 61 and 62 which is preferably equal to one-fourth of theoptical wavelength of the incident light. The optical wavelength througha layer is determined by dividing the wavelength of light in question bythe index of refraction of the medium through which it is passing, i.e.layers 61 and 62. For visible light having wavelengths between about400-650 nm, a useful approximation of optical wavelength can be made bydividing a wavelength of 540 nm by the index of refraction of theoptical tuning layer. The thickness of the optical tuning layer can thenbe determined by dividing the optical wavelength by 4. Optical tuninglayers used in combination with transparent conductive oxide layers,such as layers 20 and 22, desirably have a thickness of between about 60nm and about 90 nm.

As an example of the foregoing, the following will demonstrate themethod for selecting the index of refraction and thickness of opticaltuning layer 61. Firstly, the desired index of refraction is determinedby multiplying 1.5 (the index of refraction of glass substrate 12) times2.1 (the index of refraction of transparent conductive oxide layer 20)and taking the square root of the product. This calculation yields anindex of refraction of 1.77. The thickness of optical tuning layer 61 isthen determined by dividing 540 nm by 1.77 to yield an opticalwavelength of about 300 nm. Preferably, the thickness of optical tuninglayer 61 is one-fourth of this value, or about 75 nm. The calculationfor optical tuning layer 62 is performed in the same fashion.

Preferred materials for forming the discrete optical tuning layers ofthe present invention include transparent oxides, transparent nitrides,and a combination of transparent oxides and transparent nitrides.Particularly preferred are mixtures of the oxides of silicon and tin. Inthese mixtures, the silicon to tin ratio determines the index ofrefraction, with a higher silicon to tin ratio lowering the index ofrefraction and a lower silicon to tin ratio increasing the index ofrefraction. Other materials which can be used for the optical tuninglayers are set forth in Gordon, U.S. Pat. Nos. 4,187,336 and 4,308,316,the disclosures of which are hereby incorporated herein. An appropriateselection of optical tuning layers 61 and 62 will minimize the opticalinterference at the interface between conductive oxide layer 20 andsubstrate 12 in the case of tuning layer 61, and at the interfacebetween conductive oxide layer 22 and laminate layer 70 in the case oftuning layer 62.

Laminate layer 70 is an adhesive layer for adhering glass superstrate 14to optical tuning layer 62. So that laminate layer 70 does not degradethe optical transmissivity of window 110, laminate layer 70 ispreferably selected to have an index of refraction which issubstantially the same as the index of refraction of glass superstrate14, thereby minimizing any interference effects between these layers. Inthis regard, particularly suitable adhesives for laminate layer 70 areethylene vinylacetate and polyvinylbutyral.

The electrochromic structure of window 110 is further distinguished fromthat of window 100 and prior art electrochromic devices in that theprinciple of optical tuning has been applied to ion conducting layer 41.In this case, optical interference at the interfaces between ionconducting layer 41 and the adjacent electrochromic electrode layer 30and counterelectrode layer 50 can be substantially eliminated by formingionic conducting layer 41 from a transparent material having an index ofrefraction which is approximately equal to the indices of refraction ofelectrochromic electrode layer 30 and counterelectrode layer 50. Theindices of refraction of electrochromic electrode layer 30, ionconducting layer 41 and counterelectrode layer 50 may be adjusted bymixing the materials for forming these layers with materials havinghigher or lower indices of refraction to obtain the desired result. Forexample, where it is desired to provide an ion conducting layer 41having an index of refraction greater than that provided by lithiumsilicate, mixtures of lithium silicate and titanium or zirconium may beused. When the indices of refraction of all three of these layers areapproximately equal, interference at these interfaces will besubstantially eliminated and optical transmission will be maximized.

In highly preferred embodiments, the indices of refraction ofelectrochromic electrode layer 30, ion conducting layer 41 andcounterelectrode layer 50 are adjusted to be substantially similar tothe indices of refraction of conductive oxide layers 20 and 22.Furthermore, because of the complementary nature of the above opticaltuning processes, the thicknesses of layers 20 through 22 are no longercritical with respect to optical interference, thereby providing a muchwider thickness tolerance during the formation of these layers.

FIG. 5 shows the transmission of optical energy as a function ofwavelength for a window 110 in the bleached state in which glasssubstrate 12 has an index of refraction of 1.5, optical tuning layer 61has an index of refraction of 1.77 and a thickness of 75 nm, transparentconductive oxide layer 20 has an index of refraction of 2.1 and athickness of 450 nm, electrochromic electrode layer 30 has an index ofrefraction of 2.1 and a thickness of 300 nm, ion conducting layer 41 hasan index of refraction of 2.0 and a thickness of 200 nm,counterelectrode layer 50 has an index of refraction of 2.0 and athickness of 200 nm, transparent conductive oxide layer 22 has an indexof refraction of 2.1 and a thickness of 450 nm, optical tuning layer 62has an index of refraction of 1.77 and a thickness of 75 nm, laminatelayer 70 has an index of refraction of 1.5, and glass superstrate 14 hasan index of refraction of 1.5. As clearly shown in FIG. 5, the foregoingcharacteristics produce a window 110 which exhibits excellent uniformitywith respect to optical transmission.

A window 120 in accordance with an alternate embodiment of the presentinvention is shown schematically in FIG. 6. Window 120 is substantiallythe same as window 110 described above, except that optical tuning layer61 has been replaced with a pair of optical tuning layers 63 and 64, andoptical tuning layer 62 has been removed. The use of two optical tuninglayers 63 and 64 provides advantages over the use of a single opticaltuning layer. A first advantage is that two optical tuning layersprovides an opportunity for more effective optical tuning to be achievedwith a combined tuning layer thickness which is less than the thicknessrequired for a single optical tuning layer. Another advantage to the useof two optical tuning layers is that each layer can be formed from moresimple materials than can a single optical tuning layer, therebyavoiding the complex chemistry required to form such single layers.Techniques using two layers for optical tuning are disclosed in Gordon,U.S. Pat. Nos. 4,377,613 and 4,419,386, the disclosures of which arehereby incorporated herein.

Where two optical tuning layers 63 and 64 are used adjacent to oneanother, optical tuning may be achieved by selecting an optical tuninglayer 64 which has an index of refraction which is significantly smallerthan the index of refraction of layer 20, and by selecting an opticaltuning layer 63 having an index of refraction which is significantlylarger than the index of refraction of optical tuning layer 64, and mostpreferably which is substantially similar to the index of refraction oftransparent conductive oxide layer 20. In such combination, theinterference between the various interfaces will substantially canceland transmission through the various layers will be maximized whenlayers 63 and 64 have appropriate thicknesses. Preferably, layer 64 willhave an index of refraction between about 1.4 and 1.7 and layers 63 and64 will have a combined thickness of between about 30 nm and about 70nm, with layer 64 having a greater thickness than layer 63.

FIG. 7 shows the optical transmission through window 120 in the bleachedstate as a function of wavelength, where glass substrate 12 has an indexof refraction of 1.5, optical tuning layer 63 has an index of refractionof 2.1 and a thickness of 20 nm, optical tuning layer 64 has an index ofrefraction of 1.5 and a thickness of 29 nm, transparent conductive oxidelayer 20 has an index of refraction of 2.1 and a thickness of 450 nm,electrochromic electrode layer 30 has an index of refraction of 2.1 anda thickness of 300 nm, ion conducting layer 41 has an index ofrefraction of 2.0 and a thickness of 200 nm, counterelectrode layer 50has an index of refraction of 2.0 and a thickness of 200 nm, transparentconductive oxide layer 22 has an index of refraction of 2.1 and athickness of 450 nm, laminate layer 70 has an index of refraction of1.5, and glass superstrate 14 has an index of refraction of 1.5. As canbe seen from this figure, the optical transmission through window 120 ismore uniform than through windows formed with no optical tuning layers(such as window 10), but is less uniform than through window 110. Theprimary reason for this inferior performance is the absence of anoptical tuning layer between conductive oxide layer 22 and laminatelayer 70 in the back end of the device.

An improvement in the optical transmission properties of window 120 canbe achieved by adjusting the indices of refraction of electrochromicelectrode layer 30, ion conducting layer 41 and counterelectrode layer50 so that they are more similar to one another and to the indices ofrefraction of transparent conductive oxide layers 20 and 22.

A further slight improvement in the optical transmission properties ofwindow 120 can be achieved by optimizing the index of refraction oflaminate layer 70. This can be achieved by using a laminate layer 70having an index of refraction which is the geometric mean of the indicesof refraction of the transparent conductive oxide layer 22 and the glasssuperstrate 14. Since laminate layer 70 serves to adhere superstrate 14to transparent conductive oxide layer 22, there is little opportunity tovary the thickness of this layer.

Rather than the foregoing methods for optically tuning transparentconductive oxide layer 20, this layer may be optically tuned by the useof two or more optical tuning layers selected so that the indices ofrefraction of the various layers gradually increase from the index ofrefraction of substrate 12 to the index of refraction of transparentconductive oxide layer 20. In that regard, layers 63 and 64 of window120 may be replaced by two or more layers of monotonically increasingindex of refraction. For example, for a device including a glasssubstrate 12 having an index of refraction of 1.5 and a transparentconductive oxide layer 20 having an index of refraction of 1.9, opticaltuning layer 63 will desirably have an index of refraction of about(1.5² ×1.9)^(1/3) or 1.62, and optical tuning layer 64 will desirablyhave an index of refraction of about (1.5×1.9²)^(1/3) or 1.76. Thethicknesses of layers 63 and 64 are desirably about 60 nm and 53 nm,respectively, as determined by optical modeling to achieve maximumtransmission. Where desirable, a series of more than two adjacentoptical tuning layers may be used to further enhance the optical tuningaffect.

For the embodiments of the invention having the structure of window 110or window 120 described above, it has been desirable that thetransparent conductive oxide layers 20 and 22, the electrochromicelectrode layer 30, the ion conducting layer 41, and thecounterelectrode layer 50 all have about the same index of refraction.When that is the case, the thicknesses of these layers are largelyimmaterial and optimum optical tuning is achieved, i.e., these windowsexhibit a minimum amount of iridescence. If there is a substantialmismatch in the indices of refraction for these layers, however, someoptical tuning may still be accomplished by adjusting the thicknesses ofthe layers. To illustrate, each of the layers of window 120 may have theindices of refraction and thicknesses as described above in connectionwith FIG. 7, except that ion conducting layer 41 may have an index ofrefraction of 1.5. To minimize iridescence in this scenario, thethickness of ion conducting layer 41 should be reduced to between about130 nm and about 160 nm as determined by optical modeling to achievemaximum transmission through the window.

A window 130 in accordance with a still further embodiment of thepresent invention is shown in schematic cross-section in FIG. 8. Window130 is similar to window 120 described above, with the exception thattransparent conductive oxide layer 22 has been replaced with aconductive metal layer 21 and an optical tuning layer 64. Layers 21 and64 may be substantially similar to layers 21 and 60 of window 100described above, and coact with the other layers in the same manner asin window 120 to provide an optically tuned, optically transparentelectrochromic window.

FIG. 9 shows the optical transmission as a function of wavelength in thebleached state of a window 130 whose layers have the same indices ofrefraction and thicknesses as described above in connection with window120, and wherein conductive metal layer 21 is silver having a thicknessof 10 nm and optical tuning layer 64 is a transparent oxide having anindex of refraction of 2.5 and a thickness of 30 nm. As can be seen fromthis figure, the structure of window 130 provides good uniformity ofoptical transmission with respect to wavelength. Moreover, a comparisonof FIGS. 7 and 9 indicates that windows that incorporate conductivemetal layers can exhibit optical transmission which is similar to theoptical transmission through windows which utilize transparentconductive oxide layers rather than conductive metal layers.

Still another embodiment of a window 140 in accordance with the presentinvention is shown schematically in FIG. 10. Window 140 is substantiallythe same as window 130 described above, except that an intermediatelayer 22 has been added between conductive metal layer 21 andcounterelectrode layer 50. In one embodiment, intermediate layer 22 isformed from a transparent electrically conductive oxide, combining withconductive metal layer 21 to provide a laminate having a lower overallsheet resistance which will improve switching time between the bleachedand colored states. This improved switching can thus be accomplishedwithout deteriorating the optical properties of the device as wouldhappen if the thickness of the conductive metal layer 21 weresubstantially increased. Further, transparent electrically conductiveoxide layer 22 may serve as a diffusion barrier to prevent unwanteddiffusion between the counterelectrode layer 50 and the conductive metallayer 21.

In another embodiment, intermediate layer 22 may be in the form of ametal layer having a thickness of less than about 2 nm. In thisembodiment, the metal layer typically serves one of the same functionsas layers formed from transparent electrically conductive oxides, thatis it prevents unwanted diffusion between the counterelectrode layer 50and the conductive metal layer 21. Preferred metals for forming layer 22include metals which are highly stable or inert. A particularlypreferred metal in that regard is nickel. Regardless of the material forforming intermediate layer 22, optical tuning layer 64 again preferablyhas an index of refraction of greater than 1.9 and a thickness ofbetween about 10 nm and about 50 nm.

A window 150 in accordance with yet another embodiment of the presentinvention is shown schematically in FIG. 11. Window 150 is substantiallythe same as window 130, except that transparent oxide layer 20 andoptical tuning layers 63 and 64 have been replaced by conductive metallayer 23 and optical tuning layer 65, respectively, thereby placing aconstraint on the combined thickness of layers 30, 41 and 50. It isdesirable in this embodiment that layers 30, 41 and 50 have similarindices of refraction and that the combined thickness of these layers beas thin as possible, subject to the restraint of a maximum transmissionat a chosen wavelength between about 400 nm and 650 nm. For example, iflayers 30, 41 and 50 each have indices of refraction of about 2.2, thentheir combined thickness preferably would be about 50 nm or about 300 nmas determined by optical modeling to achieve maximum opticaltransmission. In this embodiment, conductive metal layer 23 is similarto conductive metal layer 21, and optical tuning layer 65 is similar tooptical tuning layer 64.

In another embodiment of the invention, (not shown) optical tuning layer64 of window 130, 140 or 150 may be formed from an electricallyconductive transparent oxide. In this event, the manufacture of thesedevices may be simplified by connecting wires 6 and 8 to layer 64 ratherthan to conductive metal layer 21, thereby avoiding the masking oretching steps which would otherwise be required.

As discussed above, the electrochromic devices of the present inventionare typically built upon a substrate, such as glass substrate 12. Thissubstrate not only supports the very thin layers of these devices duringfabrication and use, but also protects the layers from damage resultingfrom exposure to the environment. A superstrate, such as glasssuperstrate 14, provides further support and protection. Although eachof the embodiments of the present invention have been described inconnection with a substrate and a superstrate made from glass, othersubstrate and superstrate materials may be used, including transparentceramic materials and rigid and flexible transparent plastics. In thatregard, the embodiment shown in FIG. 11 is particularly applicable toplastic substrates since the conductive metal layer 23 can be applied atrelatively low temperatures. Furthermore, it is contemplated thatelectrochromic devices may initially be formed on plastic substrates andthe entire assembly then applied to windows and other such structures.

Furthermore, the embodiments of the present invention discussed abovecontemplate the deposition of layers successively from substrate 12 tosuperstrate 14 using ion conducting layers formed from ceramicmaterials. It will be appreciated, however, that optically tuned devicesmay be formed by assembling two "one-half" structures to opposite sidesof a polymer sheet which will then serve as the ion conducting layer.For example, window 110 can be produced by forming layers 61, 20 and 30on substrate 12, forming layers 62, 22 and 50 on superstrate 14 (layer70 would no longer be necessary in this arrangement), and thensandwiching these two portions together around an ion conducting,electrically insulating polymer sheet which will thus serve as ionconducting layer 41.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as set forth in the appended claims.

We claim:
 1. An electrochromic device, comprisingan electrochromicstructure including an elelectrochromic mate an electrochromic material,a counterelectrode and transporting means for transporting ions betweensaid electrode and said counterelectrode, conductive means including atleast two electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, at least one of said electrically conductive layers includinga layer of an electrically conductive oxide, a substrate and asuperstrate together cooperating to sandwich said electrochromicstructure and said electrically conductive layers, and enhancing meansfor enhancing the transmission of radiation through said electricallyconductive layers, said enhancing means including at least one opticallytransparent layer including silicon dioxide in surface contact with saidsubstrate and at least one other optically transparent layer includingsilicon dioxide in surface contact with said superstrate.
 2. Theelectrochromic device as claimed in claim 1, wherein said at least oneoptically transparent layer is interposed between said substrate andsaid one of said electrically conductive layers, and said at least oneother optically transparent layer is interposed between said superstrateand another of said electrically conductive layers.
 3. Theelectrochromic device as claimed in claim 1, wherein said one of saidelectrically conductive layers between said electrochromic structure andsaid substrate includes a transparent oxide and said at least oneoptically transparent layer is interposed between said substrate andsaid one of said electrically conductive layers in surface contact withsaid transparent oxide.
 4. An electrochromic combination, comprisingasubstrate, and an electrochromic device arranged on said substrate, saidelectrochromic device including an electrochromic structure having anelectrode formed from an electrochromic material, a counterelectrode,lithium ions and transporting means for transporting said lithium ionsbetween said electrode and said counterelectrode, conductive meansincluding at least two electrically conductive layers sandwiching saidelectrochromic structure for applying an electric potential across saidelectrochromic structure, at least one of said electrically conductivelayers between said electrochromic structure and said substrateincluding a transparent conductive oxide and excluding a metal layer,and enhancing means for enhancing the transmission of radiation throughsaid at least one of said electrically conductive layers, said enhancingmeans including at least one optically transparent layer in surfacecontact with said substrate.
 5. The electrochromic combination asclaimed in claim 4, wherein said at least one optically transparentlayer is interposed between said substrate and said at least one of saidelectrically conductive layers.
 6. An electrochromic combination,comprisinga substrate, an electrochromic device arranged on saidsubstrate, said electrochromic device including an electrochromicstructure having an electrode formed from an electrochromic material, acounterelectrode and transporting means for transporting ions betweensaid electrode and said counterelectrode, conductive means including atleast two electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, at least one of said electrically conductive layers betweensaid electrochromic structure and said substrate including a transparentconductive oxide and excluding a metal layer, and enhancing means forenhancing the transmission of radiation through said at least one ofsaid electrically conductive layers, said enhancing means including atleast one optically transparent layer including silicon dioxide insurface contact with said substrate, and a superstrate cooperating withsaid substrate to sandwich said electrochromic device.
 7. Anelectrochromic combination, comprisinga substrate, and an electrochromicdevice arranged on said substrate, said electrochromic device includingan electrochromic structure having an electrode formed from anelectrochromic material, a counterelectrode, lithium ions andtransporting means for transporting said lithium ions between saidelectrode and said counterelectrode, conductive means including at leasttwo electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, and enhancing means for enhancing the transmission ofradiation through said electrically conductive layers, said enhancingmeans including at least two optically transparent layers sandwichingsaid electrochromic structure and said electrically conductive layers,one of said optically transparent layers being in surface contact withsaid substrate.
 8. The electrochromic combination as claimed in claim 7,wherein said at least two optically transparent layers have an index ofrefraction greater than about 1.5.
 9. The electrochromic combination asclaimed in claim 7, wherein said one of said optically transparentlayers is interposed between said substrate and said one of saidelectrically conductive layers.
 10. An electrochromic combination,comprisinga substrate, an electrochromic device arranged on saidsubstrate, said electrochromic device including an electrochromicstructure having an electrode formed from an electrochromic material, acounterelectrode and transporting means for transporting ions betweensaid electrode and said counterelectrode, conductive means including atleast two electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, and enhancing means for enhancing the transmission ofradiation through said electrically conductive layers, said enhancingmeans including at least two optically transparent layers sandwichingsaid electrochromic structure and said electrically conductive layers,one of said optically transparent layers being in surface contact withsaid substrate, at least one of said optically transparent layersincluding silicon dioxide, and a superstrate cooperating with saidsubstrate to sandwich said electrochromic device.
 11. The electrochromiccombination as claimed in claim 10, wherein said one of said opticallytransparent layers is interposed between said substrate and said one ofsaid electrically conductive layers, and another one of said opticallytransparent layers is interposed between said superstrate and another ofsaid electrically conductive layers.
 12. An electrochromic combination,comprisinga substrate, and an electrochromic device arranged on saidsubstrate, said electrochromic device including an electrochromicstructure having an electrode formed from an electrochromic material, acounterelectrode, lithium ions and transporting means for transportingsaid lithium ions between said electrode and said counterelectrode,conductive means including at least two electrically conductive layerssandwiching said electrochromic structure for applying an electricpotential across said electrochromic structure, at least one of saidelectrically conductive layers between said electrochromic structure andsaid substrate including a transparent oxide, and enhancing means forenhancing the transmission of radiation through said at least one ofsaid electrically conductive layers, said enhancing means including atleast one optically transparent layer interposed between said substrateand said at least one of said electrically conductive layers in surfacecontact with said transparent oxide.
 13. An electrochromic combination,comprisinga substrate, an electrochromic device arranged on saidsubstrate, said electrochromic device including an electrochromicstructure having an electrode formed from an electrochromic material, acounterelectrode and transporting means for transporting ions betweensaid electrode and said counterelectrode, conductive means including atleast two electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, at least one of said electrically conductive layers betweensaid electrochromic structure and said substrate including a transparentoxide, and enhancing means for enhancing the transmission of radiationthrough said at least one of said electrically conductive layers, saidenhancing means including at least one optically transparent layerincluding silicon dioxide interposed between said substrate and said atleast one of said electrically conductive layers in surface contact withsaid transparent oxide, and a superstrate cooperating with saidsubstrate to sandwich said electrochromic device.
 14. The electrochromiccombination as claimed in claim 4, further comprising a superstratecooperating with said substrate to sandwich said electrochromic device.15. The electrochromic combination as claimed in claim 7, furthercomprising a superstrate cooperating with said substrate to sandwichsaid electrochromic device.
 16. The electrochromic combination asclaimed in claim 12, further comprising a superstrate cooperating withsaid substrate to sandwich said electrochromic device.
 17. Anelectrochromic combination, comprisinga substrate, and an electrochromicdevice arranged on said substrate, said electrochromic device includingan electrochromic structure having an electrode formed from anelectrochromic material, a counterelectrode and transporting meansconsisting of only a single layer for transporting ions between saidelectrode and said counterelectrode, conductive means including at leasttwo electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, at least one of said electrically conductive layers betweensaid electrochromic structure and said substrate including a transparentconductive oxide and excluding a metal layer, and enhancing means forenhancing the transmission of radiation through said at least one ofsaid electrically conductive layers, said enhancing means including atleast one optically transparent layer including silicon dioxide insurface contact with said substrate.
 18. An electrochromic combination,comprisinga substrate, and an electrochromic device arranged on saidsubstrate, said electrochromic device including an electrochromicstructure having an electrode formed from an electrochromic material, acounterelectrode and transporting means consisting of only a singlelayer for transporting ions between said electrode and saidcounterelectrode, conductive means including at least two electricallyconductive layers sandwiching said electrochromic structure for applyingan electric potential across said electrochromic structure, andenhancing means for enhancing the transmission of radiation through saidelectrically conductive layers, said enhancing means including at leasttwo optically transparent layers sandwiching said electrochromicstructure and said electrically conductive layers, one of said opticallytransparent layers being in surface contact with said substrate, atleast one of said optically transparent layers including silicondioxide.
 19. An electrochromic combination, comprisinga substrate, andan electrochromic device arranged on said substrate, said electrochromicdevice including an electrochromic structure having an electrode formedfrom an electrochromic material, a counterelectrode and transportingmeans consisting of only a single layer for transporting ions betweensaid electrode and said counterelectrode, conductive means including atleast two electrically conductive layers sandwiching said electrochromicstructure for applying an electric potential across said electrochromicstructure, at least one of said electrically conductive layers betweensaid electrochromic structure and said substrate including a transparentoxide, and enhancing means for enhancing the transmission of radiationthrough said at least one of said electrically conductive layers, saidenhancing means including at least one optically transparent layerincluding silicon dioxide interposed between said substrate and said atleast one of said electrically conductive layers in surface contact withsaid transparent oxide.